1. Field of the Invention
This invention relates to a screw jack used for raising a structural object or a work piece, and more particularly to a screw jack that utilizes a screw.
2. Description of the Prior Art
The conventional automatic three-stage screw jack, as shown in FIG. 6, is structurally designed to equip a ball bearing C on a bed B at a lower end of an external cylinder A to receive and support a screw shaft D on the ball bearing C. Therefore, there is a disadvantage that the jack is able to raise an object only up to a level from which the thickness of the ball bearing has subtracted.
An object of the present invention is to provide a screw jack that is able raise an object up to a higher level in comparison with the conventional one.
According to the present invention, there is provided a screw jack that comprises a thin metal bearing arranged on a bottom surface of a jack body, where the thin metal bearing receives and supports a main screw shaft on the jack body. The thin metal bearing is constructed so as to have a large outer diameter while a stepped sliding-surface is constructed so as to have a small diameter and provided on the top or bottom surface of the large-diameter metal bearing.